Semiconductor device and method of fabricating the same

ABSTRACT

A crystalline semiconductor film in which the position and size of a crystal grain is controlled is fabricated, and the crystalline semiconductor film is used for a channel formation region of a TFT, so that a high performance TFT is realized. An island-like semiconductor layer is made to have a temperature distribution, and a region where temperature change is gentle is provided to control the nucleus generation speed and nucleus generation density, so that the crystal grain is enlarged. In a region where an island-like semiconductor layer  1003  overlaps with a base film  1002 , a thick portion is formed in the base film  1002 . The volume of this portion increases and heat capacity becomes large, so that a cycle of temperature change by irradiation of a pulse laser beam to the island-like semiconductor layer becomes gentle (as compared with other thin portion). Like this, a laser beam is irradiated from the front side and reverse side of the substrate to directly heat the semiconductor layer, and heat conduction from the semiconductor layer to the side of the substrate and heat conduction of the semiconductor layer in the horizontal direction to the substrate are used, so that the increase in the size of the crystal grain is realized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating a semiconductorfilm having a crystal structure and formed on a substrate having aninsulating surface, and a method of fabricating a semiconductor deviceusing the semiconductor film as an active layer. Particularly, thepresent invention relates to a method of fabricating a thin filmtransistor in which an active layer is formed of a crystallinesemiconductor layer. Incidentally, in the present specification, theterm “semiconductor device” indicates all devices capable of functioningby using semiconductor characteristics, and includes, in its category,an electro-optical device typified by an active matrix type liquidcrystal display device formed by using thin film transistors, and anelectronic equipment incorporating that kind of electro-optical deviceas a part.

2. Description of the Related Art

There has been developed a thin film transistor (hereinafter referred toas a TFT) in which an amorphous semiconductor layer is formed on atranslucent substrate having an insulating surface and a crystallinesemiconductor layer crystallized by a laser annealing method, heatannealing method or the like is made an active layer. As the insulatingsubstrate, a glass substrate of barium borosilicate glass or aluminoborosilicate glass is often used. Although such a glass substrate isinferior to a quartz substrate in heat resistance, it has merits thatits market price is inexpensive and a large area substrate can be easilymanufactured.

The laser annealing method is known as a crystallizing technique inwhich it is possible to crystallize an amorphous semiconductor layer bygiving high energy to only the amorphous semiconductor layer withoutraising the temperature of a glass substrate very much. Particularly anexcimer laser capable of obtaining short wavelength light having awavelength of 400 nm or less and large output is regarded as mostsuitable in this usage. The laser annealing method using the excimerlaser is carried out in such a manner that a laser beam is processed byan optical system into a spot shape or linear shape on a surface to beirradiated, and the surface to be irradiated on the substrate is scannedby the processed laser beam (irradiation position of the laser beam ismoved relatively to the surface to be irradiated). For example, in anexcimer laser annealing method using a linear laser beam, it is alsopossible to make laser annealing of all the surfaces to be irradiated byscanning only in the direction normal to its longitudinal direction, andis superior in productivity, so that it has become the mainstream of amanufacturing technique of a liquid crystal display device using TFTs.The technique enables a monolithic type liquid crystal display device inwhich TFTs (pixel TFTs) for forming a pixel portion and TFTs of adriving circuit provided at the periphery of the pixel portion areformed on one glass substrate.

However, a crystalline semiconductor layer fabricated by the laserannealing method is formed of an aggregation of plural crystal grains,and the positions and sizes of the crystal grains are random. TFTsfabricated on the glass substrate are formed such that the crystallinesemiconductor layer is separated into an island-like pattern for thepurpose of element separation. In that case, it was impossible tospecify the positions and sizes of the crystal grains and form them. Inthe interface (crystal grain boundary) of the crystal grain, there is acause to lower current transport characteristics of carriers because ofa recombination center or trapping center due to an amorphous structure,crystal defect or the like, or the influence of a potential level at thecrystal grain boundary. However, it has been hardly possible to form achannel formation region, in which the property of a crystal greatlyinfluences the characteristics of a TFT, by a single crystal grain so asto exclude the influence of the crystal grain boundary. Thus, a TFTincluding an active layer of a crystalline silicon film and havingcharacteristics comparable to those of a MOS transistor has not beenobtained till today.

In order to solve such problems, an attempt to grow a large crystalgrain has been made. For example, in ┌“High-Mobility Poly-Si Thin-FilmTransistors Fabricated by a Novel Excimer Laser Crystallization Method”,K. Shimizu, O. Sugiura, and M. Matsumura, IEEE Transactions on ElectronDevices vol. 40. No. 1, pp 112-117, 1993┘, there is a report on a laserannealing method in which a film of three-layer structure of Si/SiO₂/Siis formed on a substrate, and an excimer laser beam is irradiated fromboth sides of a film side and a substrate side. This report disclosesthat according to this method, the size of a crystal grain can beenlarged by irradiation of a laser beam at predetermined energyintensity.

The above-mentioned method of Ishihara et al. is characterized in thatheat characteristics of an under material of an amorphous silicon filmare locally changed and the flow of heat to the substrate is controlled,so that a temperature gradient is caused. However, for that purpose, thethree-layer structure of high melting point metal layer/silicon oxidelayer/semiconductor film is formed on the glass substrate. Although itis possible to form a top gate type TFT by using the semiconductor filmas an active layer in view of structure, since a parasitic capacitanceis generated by the silicon oxide film provided between thesemiconductor film and the high melting point metal layer, powerconsumption is increased and it becomes difficult to realize high speedoperation of the TFT.

On the other hand, when the high melting point metal layer is made agate electrode, it is conceivable that the method can be effectivelyapplied to a bottom gate type or reverse stagger type TFT. However, inthe foregoing three-layer structure, even if the thickness of thesemiconductor film is omitted, with respect to the thickness of the highmelting point metal layer and the silicon oxide layer, since thethickness suitable for a crystallizing step is not necessarilycoincident with the thickness suitable for the characteristics as a TFTelement, it is impossible to simultaneously satisfy both the optimumdesign in the crystallizing step and the optimum design in the elementstructure.

Besides, when the opaque high melting point metal layer is formed on theentire surface of the glass substrate, it is impossible to fabricate atransmission type liquid crystal display device. Although the highmelting point metal layer is useful in that its thermal conductivity ishigh, since a chromium (Cr) film or titanium (Ti) film used as the highmelting point metal material layer has high internal stress, there is ahigh possibility that a problem as to adhesiveness to the glasssubstrate occurs. Further, the influence of the internal stress is alsoexerted on the semiconductor film formed as the upper layer, and thereis a high possibility that the stress functions as force to impartdistortion to the formed crystalline semiconductor film.

On the other hand, in order to control a threshold voltage (hereinafterreferred to as Vth) as an important characteristic parameter in a TFTwithin a predetermined range, in addition to valence electron control ofthe channel formation region, it is necessary to reduce the chargeddefect density of a base film and a gate insulating film formed of aninsulating film to be in close contact with the active layer, or toconsider the balance of the internal stress. To such requests, amaterial containing silicon as its constituent element, such as asilicon oxide film or a silicon nitride oxide film, has been suitable.Thus, there is a fear that the balance is lost by providing the highmelting point metal layer to cause the temperature gradient.

SUMMARY OF THE INVENTION

The present invention has been made to solve such problems, and anobject of the invention is to realize a TFT capable of operating at highspeed by fabricating a crystalline semiconductor film in which thepositions and sizes of crystal grains is controlled and further by usingthe crystalline semiconductor film for a channel formation region of theTFT. Further, another object of the invention is to provide a techniqueenabling such a TFT to be applied to various semiconductor devices suchas a transmission type liquid crystal display device or a display deviceusing organic electroluminecence material.

A laser annealing method is used as a method of forming a crystallinesemiconductor layer from an amorphous semiconductor layer formed on asubstrate of glass or the like. In the laser annealing method of thisinvention, a pulse oscillation or continuous-wave excimer laser or argonlaser is used as a light source, and a laser beam formed into a linearshape by an optical system is irradiated to an island-like semiconductorlayer from both sides of a front side of a substrate where theisland-like semiconductor layer is formed (in this specification, thefront side is defined as a surface where an island-like semiconductorlayer is formed) and a reverse side (in this specification, it isdefined as a surface opposite to the surface where the island-likesemiconductor layer is formed).

FIG. 2A is a view showing a structure of a laser annealing apparatus ofthe present invention. The laser annealing apparatus includes a laseroscillator 1201, an optical system 1100, and a stage 1202 for fixing asubstrate. The stage 1202 is provided with a heater 1203 and a heatercontroller 1204, and can heat the substrate up to 100 to 450° C. Areflecting plate 1205 is provided on the stage 1202, and a substrate1206 is set thereon. In the structure of the laser annealing apparatusof FIG. 2A, a method of holding the substrate 1206 will be describedwith reference to FIG. 2B. The substrate 1206 held at the stage 1202 isset in a reaction chamber 1213, and is irradiated with a laser beam. Theinside of the reaction chamber can be made a low pressure state or inertgas atmosphere by a not-shown exhaust system or gas system, and asemiconductor film can be heated up to 100 to 450° C. without pollution.The stage 1202 can be moved along a guide rail 1216 in the reactionchamber, and the entire surface of the substrate can be irradiated withthe linear laser beam. The laser beam is incident from a not-shownquartz window provided above the substrate 1206. Besides, in FIG. 2B, atransfer chamber 1210, an intermediate chamber 1211, and a load/unloadchamber 1212 are connected to the reaction chamber 1213, and they areseparated by partition valves 1217 and 1218. A cassette 1214 capable ofholding a plurality of substrates is set in the load/unload chamber1212, and the substrate is conveyed by a conveying robot 1215 providedin the transfer chamber 1210. A substrate 1206′ indicates a substrateunder conveyance. By adopting such structure, it is possible tocontinuously carry out laser annealing under the low pressure or in theinert gas atmosphere.

FIGS. 3A and 3B are views for explaining the structure of the opticalsystem 1100 of the laser annealing apparatus shown in FIG. 2A. Anexcimer laser, argon laser or the like is used as a laser oscillator1101. FIG. 3A is a view of the optical system 1100 seen from the side,and a laser beam emitted from the laser oscillator 1101 is divided inthe vertical direction by a cylindrical lens array 1102. After thedivided laser beam is once condensed by a cylindrical lens 1104, itbroadens, is reflected by a mirror 1107, and then, is made a linearlaser beam on an irradiation surface 1109 by a cylindrical lens 1108. Bythis, the energy distribution of the linear laser beam in a widthdirection can be uniformed. FIG. 3B is a view of the optical system 1100seen from above, and the laser beam emitted from the laser oscillator1101 is divided in the horizontal direction by the cylindrical lensarray 1102. Thereafter, the laser beams are synthesized into one beam onthe irradiation surface 1109 by the cylindrical lens 1105. By this, theenergy distribution of the linear laser beam in the longitudinaldirection can be uniformed.

FIG. 1 is a view for explaining the concept of a laser annealing methodof the present invention, An insulating film 1002 is formed on asubstrate 1001 of glass or the like, and an island-like semiconductorlayer 1003 is formed thereon. A silicon oxide film, a silicon nitridefilm, a silicon nitride oxide film, an insulating film containingaluminum as its main ingredient, or the like is applied to theinsulating film 1002, and a single film among these or a suitablecombination of these is used. By the optical system explained in FIGS.3A and 3B, the laser beam having passed through the cylindrical lens1005 with the function equivalent to the cylindrical lens 1108 isirradiated as the linear laser beam to the island-like semiconductorlayer 1003. The island-like semiconductor layer 1003 receives a firstlaser beam component 1006 which passes through the cylindrical lens 1005and is directly irradiated to the island-like semiconductor layer 1003and a second laser beam component 1007 which passes through theinsulating film 1002 and the substrate 1001, is reflected by areflecting plate 1004, again passes through the substrate 1001 and theinsulating film 1002, and is irradiated to the island-like semiconductorlayer 1003. In any case, since the laser beam having passed through thecylindrical lens 1005 has an incident angle of 45 to 90° with respect tothe surface of the substrate in the condensing process, the laser beamreflected by the reflecting plate 1004 is also reflected toward theinside of the island-like semiconductor layer 1003. In the reflectingplate 1004, a reflecting surface is formed of aluminum (Al), titanium(Ti), titanium nitride (TiN), chromium (Cr), tungsten (W), tungstennitride (WN), or the like. Like this, by suitably selecting the materialforming the reflecting surface, the reflectivity can be changed in therange of 20 to 90%, and the intensity of the laser beam incident fromthe reverse side of the substrate 1001 can be changed. If the reflectionsurface is made a mirror surface, positive reflectivity of about 90% canbe obtained within the wavelength range of 240 to 320 nm. Besides, ifthe material is made aluminum and minute uneven shapes of severalhundred nm are formed on the surface, diffusion reflectivity (integralreflectivity—positive reflectivity) of 50 to 70% is obtained.

In this way, the laser beam is irradiated from the front surface and thereverse surface of the substrate 1001, and the island-like semiconductorlayer formed on this substrate 1001 is laser annealed from bothsurfaces. In the laser annealing method, by optimizing the condition ofan irradiated laser beam, a semiconductor film is instantaneously heatedand melted, and the generation density of crystal nuclei and crystalgrowth from the crystal nuclei is controlled. Since the oscillationpulse width of an excimer laser is several nano seconds to several tensnano seconds, for example, 30 nano seconds, if irradiation is made undera pulse oscillation frequency of 30 Hz, the semiconductor layer of theregion which is irradiated with the laser beam is instantaneously heatedby the pulse laser beam, and is cooled for a time far longer than theheating time.

If the laser beam is irradiated to the island-like semiconductor layerformed on the substrate from only one surface, only one side is heated,so that a cycle of heating melting and cooling solidification becomessteep. Thus, even if the generation density of crystal nuclei can becontrolled, satisfactory crystal growth can not be expected. However, ifthe laser beam is irradiated from both surfaces of the semiconductorlayer, the cycle of heating melting and cooling solidification becomesgentle, and a time allowed for crystal growth in the process of coolingsolidification becomes relatively long, so that satisfactory crystalgrowth can be obtained.

In the transient phenomenon, an attempt is made such that theisland-like semiconductor layer is made to have a temperaturedistribution, a region where temperature change is gentle is formed, anda nucleus generation speed and nucleus generation density arecontrolled, so that the size of the crystal grain is enlarged.Specifically, as shown in FIG. 1, in the region where the island-likesemiconductor layer 1003 overlaps with the base film 1002, a thickportion is formed in the base film 1002. At this portion, since itsvolume is increased and heat capacity is increased, the cycle oftemperature change of the island-like semiconductor layer by theirradiation of the pulse laser beam becomes gentle (as compared with theother thin portion). In the present invention, like this, the laser beamis irradiated from the front surface side and the reverse surface sideof the substrate to directly heat the semiconductor layer, and at thesame time, heat conduction control from the semiconductor layer to thesubstrate side and heat conduction (due to a temperature gradient) ofthe semiconductor layer in the horizontal direction to the substrate areused, so that enlargement of the size of the crystal grain is realized.

In addition, with respect to the method of irradiating the laser beamfrom the front surface side and the reverse surface side of thesubstrate on which the island-like semiconductor layer is formed, astructure shown in FIG. 4 may be used. A light beam emitted from a laseroscillator 401 such as an excimer laser is divided by a cylindrical lensarray 402 (or 403). After this divided laser beam is once condensed by acylindrical lens 404 (or 405), it broadens and is reflected by a mirror408. A beam splitter 406 is put on the midway of this optical path todivide the optical path in two. One laser beam is reflected by mirrors407 and 413, is made a linear laser beam by a cylindrical lens 414, andis irradiated to the front side of a substrate 418. This laser beam ismade a first laser beam. A base film 419 and an island-likesemiconductor layer 420 are formed on the front side of the substrate418. The other laser beam is reflected by mirrors 408, 409 and 411, ismade a linear laser beam by a cylindrical lens 412, and is irradiated tothe reverse side of the substrate 418. This laser beam is made a secondlaser beam. In the midway of this optical path, an attenuator isprovided to adjust the intensity of the laser beam. In this structure,even when the laser beam is irradiated from the front side and thereverse side of the substrate, the size of the crystal grain of thesemiconductor layer can be enlarged similarly to the foregoing.

In this invention, such a laser annealing method is called a dual beamlaser annealing method, and this method is used to enlarge the size of acrystal grain of an island-like semiconductor layer. Such an island-likesemiconductor layer is used for an active layer of a TFT, and further, asemiconductor device including a TFT having a structure in accordancewith the function of each circuit is fabricated, so that the performanceof the semiconductor device is improved.

The structure of the present invention using such a dual beam laserannealing method is characterized in that a base film having a region ofa first thickness and a region of a second thickness smaller than thefirst thickness are formed on one surface of a translucent substrate,the region of the first thickness has an area smaller than the region ofthe second thickness, and an island-like semiconductor layer having acrystal structure on the base film is formed over the region of thefirst thickness and the region of the second thickness.

Another structure of the invention is characterized in that a heatconduction layer formed like an island is provided on one surface of atranslucent substrate, a base film on the translucent substrate isformed to cover the heat conduction layer, and at least a part of anisland-like semiconductor layer having a crystal structure on the basefilm is formed on the heat conduction layer.

Besides, another structure of the present invention is characterized byincluding a step of forming a base film of a first thickness on onesurface of a translucent substrate, a step of forming a region of thefirst thickness and a region of a second thickness smaller than thefirst thickness by etching a part of the base film, a step of forming anisland-like semiconductor layer on the base film and over the region ofthe first thickness and the region of the second thickness, and a stepof crystallizing the island-like semiconductor layer by irradiating alaser beam to the island-like semiconductor layer from one surface sideand the other surface side of the translucent substrate.

Besides, another structure of the present invention is characterized byincluding a step of forming an island-like heat conduction layer on onesurface of a translucent substrate, a step of forming a base film of afirst thickness on the translucent substrate to cover the island-likeheat conduction layer, a step of forming an island-like semiconductorlayer which is formed on the base film, which has an area larger thanthe island-like heat conduction layer, and at least a part of whichoverlaps with the island-like heat conduction layer, and a step ofcrystallizing the island-like semiconductor layer by irradiating a laserbeam to the island-like semiconductor layer from one surface side of thetranslucent substrate and the other surface side.

Besides, another structure of the present invention is characterized byincluding a step of forming a base film of a first thickness on onesurface of a translucent substrate, a step of forming a region of afirst thickness and a region of a second thickness smaller than thefirst thickness by etching a part of the base film, a step of forming anisland-like semiconductor layer on the base film and over the region ofthe first thickness and the region of the second thickness, and a stepof crystallizing the island-like semiconductor layer by irradiating thelaser beam from one surface side of the translucent substrate and bycausing a reflecting plate provided at the other surface side of thetranslucent substrate to reflect a laser beam, which was incident on aperipheral region of the island-like semiconductor layer and passedthrough the translucent substrate, so that the laser beam is irradiatedfrom the other surface side of the translucent substrate.

Besides, another structure of the present invention is characterized byincluding a step of forming an island-like heat conduction layer on onesurface of a translucent substrate, a step of forming a base film of afirst thickness on the translucent substrate to cover the island-likeheat conduction layer, forming an island-like semiconductor layer whichis formed on the base film, which has an area larger than theisland-like heat conduction layer, and at least a part of which overlapswith the island-like heat conduction layer, and a step of crystallizingthe island-like semiconductor layer by irradiating a laser beam from onesurface side of the translucent substrate and by causing a reflectingplate provided at the other surface side of the translucent substrate toreflect a laser beam, which was incident on a peripheral region of theisland-like semiconductor layer and passed through the translucentsubstrate, so that the laser beam is irradiated from the other surfaceside of the translucent substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a view for explaining the concept of a laser annealing methodof the present invention;

FIGS. 2A and 2B are views for explaining a structure of a laserannealing apparatus;

FIGS. 3A and 3B are views for explaining a structure of an opticalsystem of the laser annealing apparatus;

FIG. 4 is a view for explaining a structure of an optical system of alaser annealing apparatus;

FIGS. 5A to 5C are views for explaining a fabricating process of anisland-like semiconductor layer of the present invention;

FIG. 6 is a view for explaining the concept of crystallization of thepresent invention.

FIGS. 7A to 7D are views for explaining a fabricating process of anisland-like semiconductor layer of the present invention;

FIGS. 8A and 8B are views for explaining the fabricating process of theisland-like semiconductor layer of the present invention;

FIG. 9 is a view for explaining the fabricating process of theisland-like semiconductor layer of the present invention;

FIGS. 10A to 10D are sectional views for explaining a fabricatingprocess of a pixel TFT and TFTs of a driving circuit;

FIGS. 11A to 11D are sectional views for explaining the fabricatingprocess of the pixel TFT and the TFTs of the driving circuit;

FIGS. 12A and 12B are views for explaining the fabricating process ofthe pixel TFT and the TFTs of the driving circuit;

FIGS. 13A to 13D are top views showing a fabricating process of a TFT ofa driving circuit

FIGS. 14A to 14D are top views showing a fabricating process of a pixelTFT;

FIG. 15 is a top view showing a pixel structure of a pixel portion;

FIGS. 16A and 16B are sectional views sowing a structure of a pixel TFT;

FIG. 17 is a sectional view showing a fabricating process of a pixel TFTand TFTs of a driver circuit;

FIG. 18 is a sectional view of an active matrix type liquid crystaldisplay device;

FIG. 19 is a top view for explaining an input/output terminal, a wiringline, a circuit arrangement, a spacer, and an arrangement of a sealingagent in a liquid crystal display device

FIG. 20 is a perspective view showing a structure of a liquid crystaldisplay device;

FIGS. 21A and 21B are views showing a structure of an active matrix typeEL display device;

FIG. 22 is a sectional view showing a structure of a pixel portion of anactive matrix type EL display device;

FIGS. 23A to 23B are views showing an example of a semiconductor device;

FIGS. 24A to 24F are views showing examples of semiconductor devices;and

FIGS. 25A to 25D are views showing structures of projection type liquidcrystal display devices;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

An embodiment of the present invention will be described with referenceto FIGS. 5A to 5C. In FIG. 5A, an alkali-free glass substrate of bariumborosilicate glass, alumino borosilicate glass, or the like is used as asubstrate 501. For example, #7059 glass or #1737 glass of Corning Inc.can be preferably used. In addition, a plastic substrate having nooptical anisotropy, such as polyethylene terephthalate (PET),polyethylene naphthalate (PEN), or polyethersulfone (PES), can also beused. On a surface of the substrate 501 on which an island-likesemiconductor layer is formed, in order to prevent the diffusion of animpurity such as an alkaline metal element from the substrate 501, abase film 502 of a silicon oxide film, a silicon nitride film, or asilicon nitride oxide film is formed to a thickness of 100 to 300 nm.The base film 502 may be formed of one layer among these films, or maybe formed by laminating the plurality of films. For example, a siliconnitride oxide film made from SiH₄, NH₃, and N₂O is formed by a plasmaCVD method.

In order to form a thick portion and a thin portion in this base film502, a resist mask is formed by a photolithography technique, and anetching process is performed. As the etching, a wet etching using asolution containing hydrofluoric acid, a dry etching using CF₄, or thelike can be applied. In any event, etching of a portion of a thicknessof 30 to 100 nm is performed to partially change the thickness of thebase film 502. FIG. 5A schematically shows the shape.

Next, an amorphous semiconductor layer 503 having a thickness of 25 to200 nm (preferably 30 to 100 nm) and an amorphous structure is formed bya well-known method such as a plasma CVD method or a sputtering method.For example, the amorphous silicon film having a thickness of 55 nm isformed by the plasma CVD method. A semiconductor film having anamorphous structure includes an amorphous semiconductor layer and amicrocrystalline semiconductor film, and a compound semiconductor filmhaving an amorphous structure, such as an amorphous silicon germaniumfilm may be applied. Then, as shown in FIG. 5B, an island-likesemiconductor layer 504 is formed from the amorphous semiconductor layer503. The island-like semiconductor layer 504 can be made square,rectangular, or arbitrarily polygonal.

Next, as shown in FIG. 5C, crystallization is performed by the dual beamlaser annealing method. The structure and concept of its apparatus isapplied similarly to that explained in FIGS. 2 to 4 as described above.For crystallization, first, it is desirable that hydrogen contained inthe amorphous semiconductor layer is eliminated in advance, and it isappropriate that a heat treatment at 400 to 500° C. for about 1 hour iscarried out to make the hydrogen content 5 atomic % or less.

Although the laser annealing condition is suitably selected by a user,for example, a pulse oscillation frequency of an excimer laser is made30 Hz, a laser energy density is made 100 to 500 mJ/cm² (typically 300to 350 mJ/cm²), and a linear beam 505 of a line width of 100 to 1000 μm,for example, a line width 400 μm is irradiated. This line width is madelarger than the island-like semiconductor layer 504, so that it ispossible to irradiate the entire surface of at least one island-likesemiconductor layer 504 at a side opposite to a substrate side and theperiphery of the island-like semiconductor layer 504 by the linear beamof one pulse. A part of light irradiated to the periphery of theisland-like semiconductor layer 504 at an incident angle θ reaches areflecting plate placed below the substrate, and a part of lightreflected at a reflection angle θ′ is irradiated to the surface of theisland-like semiconductor layer 504 at the substrate side. By using sucha linear beam, the same place is repeatedly irradiated. Alternatively,irradiation is made plural times while the linear beam is scanned. It isappropriate that an overlap ratio of the linear beams at this time ismade 50 to 98%. Actually, it is appropriate that the number ofirradiation pulses is made 10 to 40. The shape of the laser beam is notlimited to the linear shape, but even if a plane shape is adopted, thesame processing can be made.

In the laser annealing method like this, the light irradiated to theperiphery of the island-like semiconductor layer 504 at the incidentangle θ is attenuated by about 50% in the process of passing through thesubstrate 501. Even if the positive reflectivity of the reflecting plateis made 90%, it appears that the laser beam irradiated to the surface ofthe island-like semiconductor layer 504 at the substrate side is about15 to 40% of the first laser beam. However, the island-likesemiconductor layer 504 can be sufficiently heated even by the secondlaser beam of such intensity. As a result, it becomes possible tosufficiently accomplish crystal growth. Since the substrate can beheated up to 100 to 450° C. also by the heater 1203 provided in thestage 1202 shown in FIG. 2, an effect obtained by heating theisland-like semiconductor layer can be obtained to some degree. However,heating of the semiconductor layer by the second laser beam has aneffect greater than this temperature.

In order to make the second laser beam effectively incident on thecenter side of the island-like semiconductor layer 504, it is effectivethat the reflecting plate is made aluminum, minute uneven shapes ofseveral hundred nm are formed on the surface, and diffusion reflectivityis made 50 to 70% in advance. This is because a diffusion angle of thelaser beam becomes large by the surface of the minute uneven shapes.

FIG. 5C shows the state where the first laser beam 505 and the secondlaser beam 506 are irradiated to the island-like semiconductor layer.The island-like semiconductor layer can be divided into the thick region(region A) of the base film 502 and the thin region (region B) thereof.In any case, the island-like semiconductor layer is heated byirradiation of the laser beam and is put in melted state. Although it ispresumed that a crystal nucleus is generated in a cooling process wherethe melted state is shifted to a solid phase state, the nucleusgeneration density correlates with the temperature and cooling speed ofthe melted state, and a tendency that the nucleus generation densitybecomes high when the melted state is rapidly cooled from hightemperature has been obtained as empirical knowledge.

When a presumption is made on the basis of such knowledge, in the regionB where the melted state is rapidly cooled, the generation density ofcrystal nuclei becomes higher than the region A, and the crystal nucleiare generated at random, so that many crystal grains are apt to beformed, and the size of a grain becomes small by the mutual operation ofthe crystal grains grown from the respective crystal nuclei. On theother hand, in the region A, since the heat capacity is relatively largeas compared with the region B, the temperature also becomes low. As aresult, heat diffusion in the horizontal direction to the substratesurface occurs from the region B to the region A, the temperature changein the region A becomes gentle, and crystal growth is sufficientlyaccomplished. At this time, by making the nucleus generation density ofthe region A low, the size of the crystal grain can be enlarged. Fromthis, it is appropriate that the size of the region A is made about 2 to6 μm. Besides, such an effect becomes remarkable when the number ofrepeated pulses of the irradiated pulse laser beam is increased.

As a result, as shown in FIG. 5C, in an island-like semiconductor layer507 made of a crystalline semiconductor film, a large grain of 2 μm ormore is obtained with respect to the crystal grain in the region A, andin the region B, a small crystal grain as compared with that is formed.FIG. 6 is a top view showing this state, and an island-likesemiconductor layer 601 can be divided into a region A602 (inside of asquare dotted line at the center portion) and a region B606 other thanthat. The crystal growth proceeds toward the end portion of theisland-like semiconductor layer 601, with a nucleus generation region603 in the region A602 being as the center. The distance from the centerof a crystal growth end 605 can be made 1 μm or more (in FIG. 6,although it is schematically shown as a circle, an actual shape isarbitrary).

In a subsequent step, the island-like semiconductor layer 507 issubjected to a heat treatment at 300 to 450° C. in an atmospherecontaining hydrogen of 3 to 100% or a heat treatment at 200 to 450° C.in an atmosphere containing hydrogen generated by plasma, so thatremaining defects can be neutralized. When an active layer of a TFT isfabricated while the portion of the region A of the island-likesemiconductor layer 507 fabricated in this way is made a channelformation region, the characteristics of the TFT can be improved.

Embodiment 2

A method of fabricating an island-like semiconductor layer having acrystal structure which is made an active layer of a TFT is not limitedto only a laser annealing method, but both the laser annealing method ofthe present invention and a thermal annealing method may be used.Particularly, when crystallization by the thermal annealing method isapplied to a crystallizing method using a catalytic element disclosed inJapanese Patent Unexamined Publication No. Hei. 7-130652,crystallization can be realized at a temperature of 600° C. or lower.When a crystalline semiconductor layer fabricated in this way isprocessed by the laser annealing method of the present invention, acrystalline semiconductor layer of high quality can be obtained. Thisembodiment will be described with reference to FIGS. 7A to 7D.

In FIG. 7A, the glass substrate shown in the embodiment 1 can bepreferably used as a substrate 510. In addition, abase film 511 and anamorphous semiconductor layer 512 are formed similarly to theembodiment 1. A solution containing a catalytic element of 5 to 100 ppmin terms of weight is applied by a spin coating method to form a layer513 containing the catalytic element. Alternatively, the layer 513containing the catalytic element may be formed even by a sputteringmethod or an evaporation method. In that case, the thickness of thelayer 513 containing the catalytic element is made 0.5 to 2 nm. Thecatalytic element is nickel (Ni), germanium (Ge), iron (Fe), palladium(Pd), tin (Sn), lead (Pb), cobalt (Co), platinum (Pt), copper (Cu), gold(Au) or the like.

Thereafter, a heat treatment at 400 to 500° C. for about 1 hour is firstcarried out, so that the hydrogen content of the amorphous semiconductorlayer is made 5 atomic % or less. Then, a furnace for furnace annealingis used to carry out a thermal annealing in a nitrogen atmosphere at 550to 600° C. for 1 to 8 hours, preferably at 550° C. for 4 hours. By theabove steps, a crystalline semiconductor layer 514 made of a crystallinesilicon film can be obtained (FIG. 7B). When the crystallinesemiconductor layer fabricated by this heat annealing is macroscopicallyobserved by an optical microscope, it is sometimes observed that anamorphous region locally remains. In such a case, according to the Ramanspectroscopy, an amorphous component having a broad peak at 480 cm⁻¹ isobserved similarly. However, such an amorphous region can be easilyremoved by the dual beam laser annealing method of the presentinvention, and an excellent crystalline semiconductor layer can beobtained.

As shown in FIG. 7C, an island-like semiconductor layer 515 is formedfrom the crystalline semiconductor layer 514. To the substrate in thisstate, as shown in FIG. 7D, the dual beam laser annealing is carried outsimilarly to the embodiment 1. As a result, an island-like semiconductorlayer 518 having a crystal structure is newly formed after a meltingstate is once formed by a first laser beam 516 and a second laser beam517. As compared with the island-like semiconductor layer 507 explainedin FIGS. 5A to SC, in the island-like semiconductor layer 518 fabricatedin this way, a crystal grain of a comparable size or larger size can befabricated in the region A as the center. However, the catalytic elementof about 1×10¹⁷ to 1×10¹⁹/cm³ is contained in the island-likesemiconductor layer 518.

Embodiment 3

The crystallizing method of a semiconductor layer by the dual beam laserannealing method of the present invention is characterized in that thesemiconductor layer formed into an island shape is made to have atemperature distribution, the region B rapidly cooled from the meltedstate and the region A in which the heat capacity of the under layer islarge and which is gently cooled, are formed as explained in FIGS. 5 to7, and a crystal of a large grain size is grown in the region A.Although the embodiment 1 and the embodiment 2 shows examples in whichthe thickness of the base film is changed to form the regions, such astructure can also be realized by using other methods.

FIGS. 8A and 8B show an example of such methods. A heat conduction layer521 made of tantalum (Ta), Ti, Cr, W, or the like and having a thicknessof 30 to 100 nm is formed into an island shape on a substrate 520 ofglass or the like set forth in the embodiment 1. A base film 522provided thereon is not subjected to an etching process, and anamorphous semiconductor layer 523 is laminated. After an island-likesemiconductor layer is formed from the amorphous semiconductor layer523, a first laser beam 524 and a second laser beam 525 are irradiatedby the dual beam laser annealing method, so that a similar crystallinesemiconductor layer 526 can be obtained. In the crystallinesemiconductor layer 526, a region where the heat conduction layer 521 isformed corresponds to the region A, and the other portion corresponds tothe region B.

It is desirable that the heat conductivity of the heat conduction layeris 10 Wm⁻¹K⁻¹ or higher. As such a material, an oxide of aluminum(aluminum oxide (Al₂O₃)) has a heat conductivity of 20 Wm⁻¹K⁻¹ and issuitable. The aluminum oxide is not limited to a stoichiometric ratio,but other elements may be added to control the heat conductivitycharacteristics and the characteristics of internal stress or the like.For example, nitrogen is made to be contained in aluminum oxide andaluminum nitride oxide (AlN_(x)O_(1−x):0.02≦x≦0.5) may be used, or anitride of aluminum (AlN_(x)) can also be used. Besides, a compound ofsilicon (Si), oxygen (O), nitrogen (N), and M (M is at least oneselected from aluminum (Al) and rare earth elements) can be used. Forexample, AlSiON, LaSiON, or the like can be preferably used. Inaddition, boron nitride or the like can also be used. All the aboveoxide, nitride, and compounds can be formed by a sputtering method. Thiscan be formed by using a target of desired composition and by using aninert gas such as argon (Ar) or nitrogen to perform sputtering.

FIG. 9 shows an example in which instead of the heat conduction layer521 of FIGS. 8A and 8B, a translucent heat conduction layer 527containing an aluminum oxide film, an aluminum nitride film, or analuminum nitride oxide film as its main ingredient is provided. Whensuch a structure is made, and a first laser beam 528 and a second laserbeam 529 are irradiated by the dual beam laser annealing method, asimilar crystalline semiconductor layer 530 can be obtained. Also here,in the crystalline semiconductor layer 530, a region where theinsulating layer 527 is formed corresponds to the region A, and theother portion corresponds to the region B.

As described above, in this embodiment, although there has beendescribed an example in which a method of using the temperature gradientof a semiconductor layer by providing the heat conduction layer underthe base film is applied to the dual beam laser annealing methoddescribed in the embodiment 1, such a method may be combined with theembodiment 2 and is carried out.

Examples of the present invention will next be described below.

Example 1

An example of the present invention will be described with reference toFIGS. 10A to 12B. Here, along steps, a description will be made on amethod in which an n-channel TFT (hereinafter referred to as a pixelTFT) and a holding capacitance of a pixel portion, and an n-channel TFTand a p-channel TFT of a driving circuit provided at the periphery ofthe pixel portion are fabricated at the same time.

In FIG. 10A, as a substrate 101, in addition to a glass substrate ofbarium borosilicate glass, alumino borosilicate glass, or the liketypified by #7059 glass or #1737 glass of Corning Inc., in the casewhere a step of crystallization or activation is carried out by only alaser annealing method, a plastic substrate having no opticalanisotropy, such as polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), or polyethersulfone (PES), can be used. In the casewhere the glass substrate is used, a heat treatment may be previouslycarried out at a temperature lower than glass distortion point by about10 to 20° C.

Then, in order to prevent diffusion of an impurity from the substrate101, a base film 102, such as a silicon oxide film, a silicon nitridefilm, or a silicon nitride oxide film, is formed on the surface of thesubstrate where an island-like semiconductor layer as an active layer ofa TFT is to be formed. For example, a silicon nitride oxide film 102 aof 10 to 100 nm formed by a plasma CVD method from SiH₄, NH₃, and N₂Oand a hydrogenated silicon nitride oxide film 102 b of 100 to 200 nmsimilarly formed from SiH₄ and N₂O are laminated. Like this, althoughthe base film 102 may be made a two-layer structure, one layer of theabove materials may be formed, or a laminate structure of more than twolayers may be formed. In any event, the film is formed to a thickness ofabout 100 to 300 nm.

The silicon nitride oxide film is formed by using a conventionalparallel plate type plasma CVD method. With respect to the siliconnitride oxide film 102 a, SiH₄ of 10 SCCM, NH₃ of 100 SCCM, and N₂O of20 SCCM were introduced into a reaction chamber, and substratetemperature of 325° C., reaction pressure of 40 Pa, discharge powerdensity of 0.41 W/cm², discharge frequency of 60 MHz were used. On theother hand, with respect to the hydrogenated silicon nitride oxide film102 b, SiH₄ of 5 SCCM, N₂O of 120 SCCM, and H₂ of 125 SCCM wereintroduced into the reaction chamber, and substrate temperature of 400°C., reaction pressure of 20 Pa, discharge power density of 0.41 W/cm²,and discharge frequency of 60 MHz were used. These films can also becontinuously formed by only changing the substrate temperature andswitching the reaction gases. The silicon nitride oxide film 102 a isformed so that the inner stress becomes tensile stress when thesubstrate is regarded as the center. Although the silicon nitride oxidefilm 102 b is also made to have the inner stress in the same direction,the stress in the absolute value is made smaller than that of thesilicon nitride oxide film 102 a.

In order to form a thick portion and a thin portion in the base film102, a resist mask is formed by a photolithography technique, and anetching process is carried out. Although a stepped portion is determinedby the amount of etching, it is preferable to make the amountapproximately 30 to 100 nm. For example, in order to etch the siliconnitride oxide film 102 b of 150 nm by 75 nm, a wet etching using asolution containing hydrofluoric acid may be used, or a dry etchingusing CF₄ or the like can be applied. In this way, a convex shape isformed in the base film 102, and the structure schematically shown inFIG. 10A is formed. At this time, although the size of the convexportion may be suitably determined in view of the size of a TFT to befabricated, for the purpose of controlling the generation number ofcrystal nuclei, the size (diameter or length of a diagonal) of about 2to 6 μm is preferable.

Next, a semiconductor layer 103 having a thickness of 25 to 80 nm(preferably 30 to 60 nm) and an amorphous structure is formed by amethod such as a plasma CVD method or a sputtering method. For example,the amorphous silicon film having a thickness of 55 nm is formed by theplasma CVD method. A semiconductor film having an amorphous structureincludes an amorphous semiconductor layer and a microcrystallinesemiconductor film, and a compound semiconductor film having anamorphous structure, such as an amorphous silicon germanium film or anamorphous silicon carbide film, may also be used.

First, island-like semiconductor layers 104′ to 108′ having first shapesas shown in FIG. 10B are formed from the semiconductor layer 103 havingthe amorphous structure by a well-known photolithography method. FIG.13A is a top view of the island-like semiconductor layers 104′ and 105′in this state, and similarly, FIG. 14A is a top view of the island-likesemiconductor layer 108′. In FIGS. 13A to 13D and FIGS. 14A to 14D,although the island-like semiconductor layer is made rectangular and isformed so that one side has a length of 50 μm or less, the shape of theisland-like semiconductor layer can be made arbitrary, and preferably,as long as the layer has such a shape that the minimum distance betweenits center and the end portion becomes 50 μm or less, any polygon orcircle may be formed. Reference characters 102 b-1 and 102 b-2 of FIG.13A and 102b-5 of FIG. 14A designate regions of convex portions of thebase film 102 formed under the respective island-like semiconductorlayers. This convex portion corresponds to the region A explained in theembodiments 1 to 3, and its periphery corresponds to the region B.

Next, a crystallizing step is carried out to the island-likesemiconductor layers 104′ to 108′ having such first shapes. As thecrystallizing step, any method explained in the embodiments 1 to 3 canbe applied. In any event, by applying the dual beam laser annealingmethod of the present invention, the island-like semiconductor layers104′ to 108′ having the first shapes of FIG. 10B can be newlycrystallized. In this case, the film is densified with thecrystallization of the amorphous silicon film and is contracted by about1 to 15%. Thus, it is conceivable that the island-like semiconductorlayer made of such a crystalline silicon film has tensile stress whenthe substrate is regarded as the center.

In the island-like semiconductor layer made of the crystallinesemiconductor layer fabricated in this way, a large crystal grain isobtained mainly in the region of the convex portion, and a crystal grainbecomes small in the vicinity of the end portion of the island-likesemiconductor layer. Thus, the characteristics of the crystal becomedeteriorated, and even if a channel formation region of a TFT is formedin this portion, the characteristics of field effect mobility or thelike become deteriorated.

If a gate electrode of a TFT is formed to extend to the region havingthe poor characteristics of the crystal like this, excellent TFTcharacteristics can not be expected. Further, there is also apossibility that an off current value (value of current flowing in anoff state of a TFT) is increased, or a current is concentrated in thisregion and heat is locally generated. Thus, as shown in FIGS. 13B and14B, in order that the gate electrode does not extend to the end of thefirst shape island-like semiconductor layer, second shape island-likelayers 104, 105 and 108 are formed. Regions 104′, 105′ and 108′indicated by dotted lines in the drawings indicate regions where thefirst shape island-like semiconductor layers existed, and are removed byetching so that the gate electrode does not overlap with at least theend portion of the region. The shape of the second shape island-likesemiconductor layers 104, 105 and 108 may be made arbitrary. The otherisland-like semiconductor layers shown in FIG. 10B are also similarlytreated.

After the second shape island-like semiconductor layers 104 to 108 areformed, a mask layer 116 having a thickness of 50 to 100 nm and made ofa silicon oxide film is formed to cover the island-like semiconductorlayers 104 to 108 by the plasma CVD method or sputtering method. To theisland-like semiconductor layers, for the purpose of controlling thethreshold voltage (Vth) of a TFT, an impurity to give a p type may beadded at a concentration of about 1×10¹⁶ to 5×10¹⁷/cm³ to all thesurfaces of the island-like semiconductor layers. As an impurity to givethe p type to a semiconductor, an element in group 13 of the periodictable, such as boron (B), aluminum (Al), or gallium (Ga), is known.Although an ion implantation method or ion doping method may be used asthe method, the ion doping method is suitable for processing a largearea substrate. In the ion doping method, diborane (B₂H₆) is used as asource gas, and boron (B) is added. Although injection of such animpurity element is not always necessary and may be omitted, it is amethod preferably used especially for restricting the threshold voltageof an n-channel TFT within a predetermined range.

In order to form an LDD region of the n-channel TFT of the drivingcircuit, an impurity element to give an n type is selectively added tothe island-like semiconductor layers 105 and 107. For that purpose,resist masks 111 to 115 are formed in advance. As an impurity element togive the n type, phosphorus (P) or arsenic (As) may be used, and here,an ion doping method using phosphine (PH₃) is used to add phosphorus(P). In the present specification, impurity regions 117 and 118 formedhere are called first low concentration n-type impurity regions, and theconcentration of phosphorus (P) in this region is made within the rangeof 2×10¹⁶ to 5×10¹⁹/cm³. The concentration is expressed by (n⁻). Animpurity region 119 is a semiconductor layer for forming a holdingcapacity of a pixel matrix circuit, and phosphorus (P) with the sameconcentration is added also in this region (FIG. 10C).

Next, a step of activating the added impurity element is carried out.Activation can be made by a heat treatment in a nitrogen atmosphere at500 to 600° C. for 1 to 4 hours, or a laser activation method. Both maybe carried out at the same time. In the case of the method of the laseractivation, a KrF excimer laser beam (wavelength of 248 nm) was used anda linear beam was formed, and under the conditions that the oscillationfrequency was 5 to 50 Hz, the energy density was 100 to 500 mJ/cm², andthe overlap ratio of the linear beam was 80 to 98%, scanning was made sothat the entire surface of the substrate where the island-likesemiconductor layers were formed was processed. Incidentally,irradiation conditions of the laser beam are not limited, but the usermay suitably determine. At this stage, the mask layer 116 is removed byetching using a solution of hydrofluoric acid or the like.

In FIG. 10D, a gate insulating film 170 is formed of an insulating filmhaving a thickness of 40 to 150 nm and containing silicon by using aplasma CVD method or sputtering method. For example, a silicon nitrideoxide film having a thickness of 120 nm is formed. Besides, in a siliconnitride oxide film formed by adding O₂ to SiH₄ and N₂O, a fixed chargedensity in the film is lowered, and it is a preferable material for thisusage. Of course, the gate insulating film 170 is not limited to such asilicon nitride oxide film, but other insulating films containingsilicon may be used as a single layer or a laminate structure. In anyevent, the gate insulating film 170 is formed to have compression stresswhen the substrate is regarded as the center.

Then, as shown in FIG. 10D, a heat resistant conductive layer forforming a gate electrode is formed on the gate insulating film 170.Although the heat resistant conductive layer may be formed of a singlelayer, a laminate structure made of plural layers, such as two layers orthree layers, may be formed, as needed. It is appropriate that the heatresistant conductive material like this is used and such a structure isadopted that a conductive layer (A) 120 made of a conductive metalnitride film and a conductive layer (B) 121 made of a metal film arelaminated. The conductive layer (B) 121 may be formed of an elementselected from Ta, Ti, molybdenum (Mo), and W, or an alloy containing theforegoing element as its main ingredient, or an alloy film of acombination of the elements (typically Mo—W alloy film, Mo—Ta alloyfilm). The conductive layer (A) 120 is formed of tantalum nitride (TaN),WN, TiN, molybdenum nitride (MoN) or the like. Besides, the conductivelayer (A) 120 may also be formed of tungsten silicide, titaniumsilicide, or molybdenum silicide. With respect to the conductive layer(B) 121, in order to lower the resistance, it is preferable to decreasethe concentration of the contained impurity, and especially, it wasappropriate that the oxygen concentration was made 30 ppm or less. Forexample, with respect to W, when the oxygen concentration is made 30 ppmor less, a specific resistance value of 20 μΩcm or less can be realized.

It is appropriate that the thickness of the conductive layer (A) 120 ismade 10 to 50 nm (preferably 20 to 30 nm), and the thickness of theconductive layer (B) 121 is made 200 to 400 nm (preferably 250 to 350nm). In the case where the gate electrode is formed of the W film, theconductive layer (A) 120 is formed of a WN film having a thickness of 50nm by a sputtering method using W as a target and introducing an argon(Ar) gas and nitrogen (N₂) gas, and the conductive layer (B) 121 isformed of a W film having a thickness of 250 nm. As another method, theW film can also be formed by a thermal CVD method using tungstenhexafluoride (WF₆). In any event, in order to use it as the gateelectrode, it is necessary to decrease the resistance, and it isdesirable to make the resistivity of the W film 20 μΩcm or less.Although the resistivity of the W film can be decreased by enlarge thecrystal grain, in the case where many impurity elements such as oxygenare contained in the W film, crystallization is blocked and theresistance is increased. From this, in the case of the sputteringmethod, when a W target of purity of 99.9999% is used, and the W film isformed by sufficiently paying attention so that an impurity is not mixedfrom a vapor phase during the film formation, the resistivity of 9 to 20μΩcm can be realized.

On the other hand, in the case where a TaN film is used for theconductive layer (A) 120 and a Ta film is used for the conductive layer(B) 121, they can be formed similarly by the sputtering method. The TaNfilm is formed by using a target of Ta and a mixed gas of Ar andnitrogen as a sputtering gas, and the Ta film is formed by using Ar asthe sputtering gas. If a suitable amount of Xe or Kr is added to thesputtering gas, the internal stress of the formed film is relieved, andpeeling of the film can be prevented. The resistivity of α-phase Ta filmis about 20 μΩcm and can be used for the gate electrode. However, theresistivity of β-phase Ta film is about 180 μΩcm and is not suitable forthe gate electrode. Since the TaN film has a crystal structure close tothe α-phase, if the Ta film is formed thereon, the α-phase Ta film canbe easily obtained. Although not shown, it is effective to form asilicon film having a thickness of about 2 to 20 nm and doped withphosphorus (P) under the conductive layer (A) 120. By this, theimprovement of adhesion of the conductive film formed thereon andprevention of oxidation are realized, and at the same time, it ispossible to prevent a trace amount of alkali metal element contained inthe conductive layer (A) 120 or the conductive layer (B) 121 fromdiffusing. In any event, it is preferable that the resistivity of theconductive layer (B) 121 is made in the range of 10 to 50 μΩcm.

Then, resist masks 122 to 127 are formed by using the photolithographytechnique, and the conductive layer (A) 120 and the conductive layer (B)121 are simultaneously etched to form gate electrodes 128 to 132 and acapacitance wiring line 133. In the gate electrodes 128 to 132 and thecapacitance wiring line 133, portions 128 a to 132 a made of theconductive layer (A) and portions 128 b to 132 b made of the conductivelayer (B) are integrally formed (FIG. 11A). The positional relationamong the island-like semiconductor layers 104 and 105 and the gateelectrodes 128 and 129 in this state is shown in a top view of FIG. 13C.Similarly, the relation among the island-like semiconductor layer 108,the gate electrode 132, and the capacitance wiring line 133 is shown inFIG. 14C. In FIGS. 13C and 14C, the gate insulating film 170 is omitted.

Although a method of etching the conductive layer (A) and the conductivelayer (B) may be suitably selected by the user, in the case where thelayer is formed of a material containing W as its main ingredient asdescribed above, it is desirable to use a dry etching method using highdensity plasma in order to carry out an etching at high speed and withhigh precision. As a method of obtaining the high density plasma, amicrowave plasma or inductively coupled plasma (ICP) etching device maybe used. For example, in the etching method of W using the ICP etchingdevice, as an etching gas, two kinds of gases of CF₄ and Cl₂ areintroduced in a reaction chamber, the pressure is made 0.5 to 1.5 Pa(preferably 1 Pa), and the high frequency (13.56 MHz) power of 200 to1000 W is applied to an inductively coupled portion. At this time, thehigh frequency power of 20 W is applied to the stage where the substrateis put, and it is charged to a negative potential by self-bias, so thata positive ion is accelerated and an anisotropic etching can be carriedout. By using the ICP etching device, even the hard metal film of W orthe like can also be etched at an etching rate of 2 to 5 nm/second. Inorder to carry out the etching without leaving the residue, it isappropriate that an etching time is increased at a ratio of about 10 to20% to carry out over etching. However, at this time, it is necessary topay attention to a selection ratio of etching to the under layer. Forexample, since the selection ratio of the silicon nitride oxide film(gate insulating film 170) to the W film is 2.5 to 3, the surface wherethe silicon nitride oxide film was exposed by such an over etchingprocess, was etched by about 20 to 50 nm and became substantially thin.

Then, in order to form an LDD region in the pixel TFT, a step of addingan impurity element to give an n type (n⁻ doping step) is carried out.Here, the impurity element to give the n type is added by an ion dopingmethod using the gate electrodes 128 to 132 as masks in a self-alignedmanner. The concentration of phosphorus (P) added as the impurity togive the n type is within the range of 1×10¹⁶ to 5×10¹⁹/cm³. In thisway, as shown in FIG. 11B, second low concentration n-type impurityregions 134 to 137 are formed in the island-like semiconductor layers.

Next, to the island-like semiconductor layers forming the n-channelTFTs, high concentration n-type impurity regions functioning sourceregions or drain regions are formed (n⁺ doping step). First, resistmasks 138 to 141 are formed, and an impurity element to give the n typeis added to form high concentration n-type impurity regions 142 to 147.Phosphorus (P) is used as the impurity element to give the n type inthis region, and an ion doping method using phosphine (PH₃) is carriedout so that its concentration becomes within the range of 1×10²⁰ to1×10²¹/cm³ (FIG. 11C).

Then, in the island-like semiconductor layers 104 and 106 for formingthe p-channel TFTs, high concentration p-type impurity regions 151 to154 as source regions or drain regions are formed. Here, the gateelectrodes 128 and 130 are used as masks, and an impurity element togive the p type is added to form the high concentration p-type impurityregions in a self-aligned manner. At this time, resist masks 148 to 150are formed to cover all the surfaces of the island-like semiconductorfilms 105, 107, and 108 for forming the n-channel TFTs. The highconcentration p-type impurity regions 151 to 154 are formed by an iondoping method using diborane (B₂H₆). The boron (B) concentration in thisregion is made 3×10²⁰ to 3×10²¹/cm³ (FIG. 11D). In the highconcentration p-type impurity regions 151 to 154, phosphorus (P) isadded in the prior steps, and the high concentration p-type impurityregions 152 and 154 contain phosphorus at a concentration of 1×10²⁰ to1×10²¹/cm³, and the high concentration p-type impurity regions 151 and153 contain phosphorus at a concentration of 1×10¹⁶ to 5×10¹⁹/cm³.However, since the concentration of boron (B) added in this step is made1.5 to 3 times as high as that of phosphorus, there occurs no problem infunctioning as the source region and drain region of the p-channel TFT.

Thereafter, as shown in FIG. 12A, a first interlayer insulating film 155is formed from gate electrodes and gate insulating films. The firstinterlayer insulating film may be formed of a silicon oxide film, asilicon nitride oxide film, a silicon nitride film, or a laminate filmof a combination of these. In any event, the first interlayer insulatingfilm 155 is formed of an inorganic insulator material. The thickness ofthe first interlayer insulating film 155 is made 100 to 200 nm. Here, inthe case where the silicon oxide film is used, the film can be formed bya plasma CVD method in which discharge is made under the conditions thattetraethyl ortho silicate (TEOS) and O₂ are mixed, the reaction pressureis made 40 Pa, the substrate temperature is made 300 to 400° C., and thehigh frequency (13.56 MHz) power density is 0.5 to 0.8 W/cm². In thecase where the silicon nitride oxide film is used, the film may beformed of a silicon nitride oxide film fabricated from SiH₄, N₂O, andNH₃ by the plasma CVD method, or a silicon nitride oxide film fabricatedfrom SiH₄ and N₂O. In this case, the film can be formed underfabricating conditions that the reaction pressure is 20 to 200 Pa, thesubstrate temperature is 300 to 400° C., and the high frequency (60 MHz)power density is 0.1 to 1.0 W/cm². Besides, a hydrogenated siliconnitride oxide film fabricated from SiH₄, N₂O, and H₂ may be used.Similarly, the silicon nitride film can be fabricated from SiH₄ and NH₃by the plasma CVD method. Such first interlayer insulating film isformed so as to have compression stress when the substrate is regardedas the center.

Thereafter, a step of activating the impurity element to give the n typeor p type, which was added at its own concentration, is carried out.This step is carried out by a thermal annealing method using a furnacefor furnace annealing. In addition, a laser annealing method or a rapidthermal annealing method (RTA method) can be used. The thermal annealingmethod is carried out in a nitrogen atmosphere containing oxygen of aconcentration of 1 ppm or less, preferably 0.1 ppm or less, at 400 to700° C., typically 500 to 600° C. In this example, a heat treatment at550° C. for 4 hours was carried out. In the case where a plasticsubstrate with a low heat resisting temperature is used as the substrate101, it is preferable to use the laser annealing method.

After the step of activation, further, a heat treatment at 300 to 450°C. for 1 to 12 hours is carried out in an atmosphere containing hydrogenof 3 to 100%, and a step of hydrogenating the second shape island-likesemiconductor layer is carried out. This step is a step of terminatingdangling bonds of 10¹⁶ to 10¹⁸/cm³ existing in the second shapeisland-like semiconductor layer by thermally excited hydrogen. Asanother means of hydrogenating, plasma hydrogenating (using hydrogenexcited by plasma) may be carried out. Besides, by a heat treatment at300 to 450° C., the island-like semiconductor layer may be hydrogenatedby diffusing hydrogen of the hydrogenated silicon nitride oxide film ofthe base film 102 and the silicon nitride oxide film of the firstinterlayer insulating film 155.

After the steps of activation and hydrogenating are ended, a secondinterlayer insulating film 156 made of organic insulating material isformed to an average thickness of 1.0 to 2.0 μm. As the organic resinmaterial, polyimide, acryl, polyamide, polyimidoamid, BCB(benzocyclobutene), or the like can be used. For example, in the casewhere polyimide of a type which is thermally polymerized afterapplication onto a substrate is used, a clean oven is used and sinteringis made at 300° C. to form the film. In the case where acryl is used, atwo-liquid type is used, and after a main material and a hardening agentare mixed, a spinner is used to apply it onto the entire surface of asubstrate, and then, preheating at 80° C. for 60 seconds is carried outby a hot plate, and further, a clean oven is used and sintering at 250°C. for 60 minutes is carried out to form the film.

The second interlayer insulating film is formed of the organic insulatormaterial, so that the surface can be excellently flattened. Besides,since the organic resin material has generally low dielectric constant,parasitic capacitance can be lowered. However, since it has ahygroscopic property and is not suitable for a protecting film, as inthis example, it is necessary to use the material in combination withthe silicon oxide film, silicon nitride oxide film, silicon nitridefilm, or the like.

Thereafter, a photomask is used to form a resist mask of a predeterminedpattern, and contact holes reaching source regions or drain regionsformed in the respective island-like semiconductor films are formed. Thecontact holes are formed by a dry etching method. In this case, a mixedgas of CF₄, O₂, and He is used as an etching gas, and the secondinterlayer insulating film 156 made of the organic resin material isfirst etched, and thereafter, an etching gas is made CF₄ and O₂, and thefirst insulating film 155 is etched. Further, in order to raise theselection ratio to the island-like semiconductor layer, an etching gasis changed to CHF₃ to etch the gate insulating film 170, so that thecontact holes can be excellently formed.

Then, a conductive metal film is formed by a sputtering method or avacuum evaporation method, a resist mask pattern is formed, and sourcewiring lines 157 to 161 and drain wiring lines 162 and 166 are formed byetching. A drain wiring line 167 indicates a drain wiring line of anadjacent pixel. Here, the drain wiring line 166 functions as a pixelelectrode. Although not shown, in this example, this electrode is wiredin such a manner that a Ti film having a thickness of 50 to 150 nm isformed, a contact to the semiconductor film forming the source or drainregion of the island-like semiconductor layer is formed, and aluminum(Al) having a thickness of 300 to 400 nm is formed to overlap with theTi film.

FIG. 13D is a top view showing, in this state, the island-likesemiconductor layers 104 and 105, the gate electrodes 128 and 129, thesource wiring lines 157 and 158, and the drain wiring lines 162 and 163.The source wiring lines 157 and 158 are connected to the island-likesemiconductor layers 104 and 105 at portions 230 and 233 throughnot-shown contact holes provided in the second interlayer insulatingfilm and the first interlayer insulating film, respectively. The drainwiring line 162 and 163 are connected to the island-like semiconductorlayers 104 and 105 at portions 231 and 232, respectively. Similarly,FIG. 14D is a top view showing the island-like semiconductor layer 108,the gate electrode 132, the capacitance wiring line 133, the sourcewiring line 161, and the drain wiring line 166. The source wiring line161 and the drain wiring line 166 are connected through contact portion234 and contact portion 235 to the island-like semiconductor layer 108,respectively.

In this state, a heat treatment is carried out to improve contact of thecontact portions between the source wiring lines 157 to 161, the drainwiring lines 162 to 166 and their respective island-like semiconductorlayers. The heat treatment is carried out by using a clean oven andwithin the range of 200 to 300° C. and 1 to 4 hours.

In this way, the substrate including the TFTs of the driving circuit andthe pixel TFT of the pixel portion on the same substrate can becompleted. In the driving circuit, a first p-channel TFT 200, a firstn-channel TFT 201, a second p-channel TFT 202, and a second n-channelTFT 203 are formed. In the pixel portion, a pixel TFT 204 and a holdingcapacitance 205 are formed. In the present specification, forconvenience, such a substrate is called an active matrix substrate.

The first p-channel TFT 200 of the driving circuit has a single drainstructure including, in the second shape island-like semiconductor film104, a channel formation region 206, source regions 207 a and 207 b, anddrain regions 208 a and 208 b, which are made of high concentrationp-type impurity regions. The first n-channel TFT 201 includes, in thesecond shape island-like semiconductor film 105, a channel formationregion 209, an LDD region 210 overlapping with the gate electrode 129, asource region 212, and a drain region 211. In this LDD region, the LDDregion overlapping with the gate electrode 129 is designated by Lov, andits length in the channel length direction was made 0.5 to 3.0 μm,preferably 1.0 to 2.0 μm. By setting the length of the LDD region in then-channel TFT in this way, a high electric field generated in thevicinity of the drain region is relieved, generation of a hot carrier isprevented, and deterioration of the TFT can be prevented. Similarly, thesecond p-channel TFT 202 of the driving circuit has a single drainstructure including, in the second shape island-like semiconductor film106, a channel formation region 213, source regions 214 a and 214 b,drain regions 215 a and 215 b, which are made of high concentrationp-type impurity regions. The second n-channel TFT 203 includes, in thesecond shape island-like semiconductor film 107, a channel formationregion 216, LDD regions 217 and 218 partially overlapping with the gateelectrode 131, a source region 220, and a drain region 219. The lengthof the region Lov overlapping with the gate electrode of the TFT wasalso made 0.5 to 3.0 μm, preferably 1.0 to 2.0 μm. The LDD region notoverlapping with the gate electrode 131 is designated by Loff, and itslength in the channel length direction was made 0.5 to 4.0 μm,preferably 1.0 to 2.0 μm. The pixel TFT 204 includes, in the island-likesemiconductor film 108, channel formation regions 221 and 222, LDDregions 223 to 225, and source or drain regions 226 to 228. The lengthof the LDD region (Loff) in the channel length direction is 0.5 to 4.0μm, preferably 1.5 to 2.5 μm. Further, the holding capacitance 205 isformed of the capacitance wiring line 133, an insulating film made ofthe same material as the gate insulating film, and a semiconductor layer229 connecting with the drain region 228 of the pixel TFT 204. In FIG.12B, although the pixel TFT 204 is made a double gate structure, asingle gate structure may be adopted, or a multi-gate structure in whicha plurality of gate electrodes are provided may be adopted.

FIG. 15 is a top view showing substantially one pixel of the pixelportion of such an active matrix substrate. A section taken along A-A′in the drawing corresponds to the sectional view of the pixel portionshown in FIG. 12B. In the pixel TFT 204, the gate electrode 132 servingalso as the gate wiring line intersects with the under island-likesemiconductor layer 108 through a not-shown gate insulating film.Although not shown, the source region, the drain region, and the LDDregion are formed in the island-like semiconductor layer. Referencenumeral 234 designates a contact portion between the source wiring line161 and the source region 226; 235, a contact portion between the drainwiring line 166 and the drain region 228. The holding capacitance 205 isformed at a region where the semiconductor layer 229 extending from thedrain region 228 of the pixel TFT 204 overlaps with the capacitancewiring line 133 through the gate insulating film.

The second shape island-like semiconductor layer formed through theforegoing steps by the dual beam laser annealing method of the presentinvention has such a structure that the size of a crystal grain isenlarged especially in the channel formation region and grain boundariesare few. The second shape island-like semiconductor layer like this isused, and the structures of the pixel TFT and the TFTs constituting eachcircuit are optimized in accordance with the specification required bythe driving circuit, so that it becomes possible to improve theoperation performance and reliability of the semiconductor device.Further, activation of the LDD region, the source region and the drainregion is facilitated by forming the gate electrode out of theconductive material having heat resistance. Then, a high quality displaydevice can be realized by such an active matrix substrate. From theactive matrix substrate fabricated in this example, a reflection typeliquid crystal display device can be fabricated.

Example 2

The active matrix substrate fabricated in the example 1 can be directlyused for a reflection type liquid crystal display device. On the otherhand, in the case of forming a transmission type liquid crystal displaydevice, it is sufficient if a pixel electrode provided for each pixel ofa pixel portion is formed of a transparent electrode. In this example, amethod of fabricating an active matrix substrate corresponding to thetransmission type liquid crystal display device will be described withreference to FIGS. 16A and 16B.

The active matrix substrate is fabricated similarly to the example 1.FIG. 16A shows a structure of a pixel portion thereof. FIG. 16A shows anexample in which a transparent conductive film is first formed on asecond interlayer insulating film, a patterning process and an etchingprocess are carried out to form a pixel electrode 171, and then, a drainwiring line 172 is formed to partially overlap with the pixel electrode171. As shown in FIG. 16B, a Ti film 172 a is formed to a thickness of50 to 150 nm, a semiconductor film for forming a source or drain regionof an island-like semiconductor layer and a contact are formed, and anAl film 172 b having a thickness of 300 to 400 nm is formed on the Tifilm 172 a. When this structure is adopted, the pixel electrode 171 isin contact with only the Ti film 172 a forming the drain wiring line172. As a result, it is possible to certainly prevent the transparentconductive material from directly coming in contact with and reactingwith Al.

As a material of the transparent conductive film, it is possible to useindium oxide (In₂O₃), indium oxide—tin oxide alloy (In₂O₃-SnO₂; ITO), orthe like, which is formed by a sputtering method or vacuum evaporationmethod. An etching process of such material is carried out by ahydrochloric acid base solution. However, etching of ITO is especiallyliable to generate the residue, and in order to improve the etchingworkability, indium oxide—zinc oxide alloy (In₂O₃-ZnO) may be used. Theindium oxide—zinc oxide alloy has features that it is excellent insurface flatness and is also excellent in heat stability as comparedwith ITO. Similarly, zinc oxide (ZnO) is also a suitable material, andfurther, in order to raise transmissivity of visible light andconductivity, zinc oxide (ZnO: Ga) added with gallium (Ga), or the likecan be used.

The structure of the driving circuit may be the same as the example 1,and in this way, the active matrix substrate corresponding to thetransmission type display device can be completed.

Example 3

In a method of fabricating an island-like semiconductor layer having acrystal structure from an island-like semiconductor layer having anamorphous structure by the dual beam laser annealing method of thepresent invention, a trace amount (about 1×10¹⁷ to 1×10¹⁹/cm³) ofcatalytic element used for crystallization remains in the island-likesemiconductor layer having the crystal structure fabricated by themethod of the embodiment 2. Of course, although a TFT can be completedeven in such a state, it is preferable to remove the remaining catalyticelement from at least a channel formation region. As one of means forremoving this catalytic element, there is a means using the getteringfunction of phosphorus (P).

A gettering process by phosphorus (P) for this object can be carried outin parallel with the activation step explained in FIG. 12A. This statewill be described with reference to FIG. 17. The concentration ofphosphorus (P) required for gettering may be comparable to the impurityconcentration of the high concentration n-type impurity region, and bythe thermal annealing of the activation step, the catalytic element canbe made to segregate from the channel formation regions of the n-channelTFT and the p-channel TFT through the concentration into the impurityregions containing phosphorus (P) (directions of arrows shown in FIG.17). As a result, the catalytic element segregates in the impurityregion, and the concentration becomes about 1×10¹⁷ to 1×10¹⁹ atoms/cm³.In the TFT fabricated in this way, its off current value is lowered andcrystallinity is excellent, so that high field effect mobility can beobtained and excellent characteristics can be achieved.

Example 4

In this example, a process of fabricating an active matrix type liquidcrystal display device from an active matrix substrate fabricated in theexample 1 will be described. As shown in FIG. 18, columnar spacers 901and 902 are formed on the active matrix substrate in the state of FIG.12B. Although a method of dispersing particles of several μm to providethe spacers may be used, the columnar spacers may be formed like this byforming a resin film on the entire surface of the substrate andpatterning this. Although the material of such columnar spacers is notrestricted, for example, NN700 made by JSR Co., Ltd. is used, and afterit is applied by a spinner, an exposure and a developing process arecarried out to form a predetermined pattern. Further, heating at 150 to200° C. is carried out by a clean oven or the like to harden it.Although the shape of the columnar spacer fabricated in this way can bechanged by conditions of the exposure and developing process, if theshape of each of the columnar spacers 901 and 902 is preferably madesuch that it is columnar and its top is flat, it is suitable in securingthe mechanical strength as a liquid crystal display panel when anopposite side substrate is fitted. Although the shape of the columnarspacer may be cylindrical or prismatic and is not particularly limited,for example, when it is cylindrical, concretely, the height H is made1.2 to 5 μm, the average radius L1 is made 5 to 7 μm, and the ratio ofthe average radius L1 to the radius L2 of the bottom portion is made1:1.5. At this time, a taper angle of a side is made ±15° or less.

Although the arrangement of the columnar spacers may be arbitrarilydetermined, preferably, as shown in FIG. 18, in the pixel portion, thecolumnar spacer 902 is formed to overlap with the contact portion 235 ofthe drain wiring line 166 (pixel electrode) to cover the portion. Sincethe flatness of the contact portion 235 is damaged and liquid crystalcomes not to be oriented well in this portion, when the columnar spacer902 is formed in the form of filling the contact portion 235 with theresin for the spacer in this way, disclination or the like can beprevented.

Thereafter, an oriented film 903 is formed. Normally, polyimide resin isused for an oriented film of a liquid crystal display device. After theoriented film is formed, a rubbing treatment is carried out so thatliquid crystal molecules are oriented with a certain constant pretiltangle. The rubbing treatment is carried out so that a region which isnot subjected to rubbing in the rubbing direction from the end portionof the columnar spacer 902 provided in the pixel portion becomes 2 μm orless. Although generation of static electricity often becomes a problemin the rubbing treatment, when the columnar spacer 901 is formed on theTFT of the driving circuit and to cover the source wiring line and thedrain wiring line, the original role of the spacer and the effect toprotect the TFT from the static electricity in the rubbing step can beobtained. In FIG. 18, although the columnar spacers 901 are dividedlyformed on the source wiring line and the drain wiring line on the TFT ofthe driving circuit, in addition, they may be formed to cover the entiresurface of the driving circuit.

A light shielding film 905, a transparent conductive film 906, and anoriented film 907 are formed on an opposite substrate 904 at theopposite side. The light shielding film 905 is formed of Ti, Cr, Al orthe like to a thickness of 150 to 300 nm. The active matrix substrate inwhich the pixel portion and the driving circuit are formed is bonded tothe opposite substrate through a sealing agent 908. A filler 909 ismixed in the sealing agent 908, and the two substrates are bonded whilea uniform interval is kept by the filler 909 and the columnar spacers901 and 902. Thereafter, a liquid crystal material 910 is injectedbetween both the substrate, and complete sealing is made by a sealingagent (not shown). As the liquid crystal material, a well-known liquidcrystal material may be used. For example, in addition to a TN liquidcrystal, it is also possible to use a thresholdless antiferroelectricmixed liquid crystal showing electro-optical response properties inwhich transmissivity is continuously changed to an electric field. Somethresholdless antiferroelectric mixed liquid crystal shows V-shapedelectro-optical response characteristics. In this way, the active matrixtype liquid crystal display device shown in FIG. 18 is completed.

FIG. 19 is a top view of an active matrix substrate, which shows thepositional relation of a pixel portion, a driving circuit portion, aspacer and a sealing agent. A scan signal driving circuit 701 and animage signal driving circuit 702 are provided as driving circuits at theperiphery of a pixel portion 700. Further, a signal processing circuit703 such as a CPU or memory may be added. These driving circuits areconnected to an external input/output terminal 710 through a connectionwiring line 711. In the pixel portion 700, a gate wiring line group 704extending from the scan signal driving circuit 701 and a source wiringline group 705 extending from the image signal driving circuit 702intersect with each other in matrix form to shape pixels, and a pixelTFT 204 and a holding capacitance 205 are provided in each pixel.

The columnar spacer 706 provided in the pixel portion corresponds to thecolumnar spacer 902 shown in FIG. 18. Although the spacer may beprovided for every pixel, it may be provided every several to severaltens pixels arranged in matrix form. That is, it is appropriate that theratio of the number of spacers to the total number of pixelsconstituting the pixel portion is made 20 to 100%. Spacers 707, 708, and709 provided at the driving circuit portion may be provided to cover theentire surface, or may be provided so that they are divided into pluralportions in conformity with the position of the source and drain wiringline of each TFT.

A sealing agent 714 is formed outside of the pixel portion 700 on thesubstrate 101, the scan signal driving circuit 701, the image signaldriving circuit 702, and the other signal processing circuit 703, andinside of the external input/output terminal 710.

The structure of such an active matrix type liquid crystal displaydevice will be described with reference to a perspective view of FIG.20. In FIG. 20, the active matrix substrate is constituted by the pixelportion 700, the scan signal driving circuit 701, the image signaldriving circuit 702, and the other signal processing circuit 703, whichare formed on the glass substrate 101. In the pixel portion 700, thepixel TFT 204 and the holding capacitance 205 are provided, and thedriving circuits provided at the periphery of the pixel portion areconstituted with a CMOS circuit as a base. The scan signal drivingcircuit 701 and the image signal driving circuit 702 are connected tothe pixel TFT 204 through the gate wiring line 132 and the source wiringline 161, respectively. A flexible printed circuit (FPC) 713 isconnected to the external input terminal 710 and is used to input animage signal or the like. The flexible printed circuit 713 is fixedwhile the adhesion strength is raised by a reinforcing resin 712, and isconnected to the respective driving circuits through the connectionwiring line 711. Although not shown, a light shielding film and atransparent electrode are provided on an opposite substrate 175.

The liquid crystal display device of such a structure can be formed byusing the active matrix substrate shown in the examples 1 to 3. Forexample, when the active matrix substrate shown in the example 1 isused, a reflection type liquid crystal display device can be obtained,and when the active matrix substrate shown in the example 2 is used, atransmission type liquid crystal display device can be obtained.

Example 5

In this example, with reference to FIGS. 21A and 21B, a description willbe made on an example in which the present invention is applied to adisplay device (organic EL display device) using an active matrix typeorganic electroluminescence (organic EL) material. FIG. 21A is a circuitdiagram of an active matrix type organic display device in which adisplay region and a driving circuit at the periphery thereof areprovided on a glass substrate. This organic EL display device isconstituted by a display region 11, an X-direction peripheral drivingcircuit 12, and a Y-direction peripheral driving circuit 13, which areprovided on the substrate. This display region 11 is constituted by aswitching TFT 30, a holding capacitance 32, a current controlling TFT31, an organic EL element 33, X-direction signal lines 18 a and 18 b,power source lines 19 a and 19 b, and Y-direction signal lines 20 a, 20b and 20 c, and the like.

FIG. 21B is a top view showing substantially one pixel. It isappropriate that the switching TFT 30 is formed in the same way as then-channel TFT 204 shown in FIG. 10C, and the current controlling TFT 31is formed in the same way as the n-channel TFT 201.

FIG. 22 is a sectional view taken along line B-B′ of FIG. 21B, whichshows the switching TFT 30, the holding capacitance 32, the currentcontrolling TFT 31, and an organic EL element portion. In FIG. 22,island-like semiconductor layers 43 and 44 are fabricated by the methodof the embodiments 1 to 4. Then, base films 41 and 42, a gate insulatingfilm 45, a first interlayer insulating film 46, gate electrodes 47 and48, a capacitance wiring line 49, source and drain wiring lines 18 a, 19a, 51, 52, and a second interlayer insulating film 50 are fabricated ona substrate 40 in the same way as the example 1. Then, similarly to thesecond interlayer insulating film 50, a third interlayer insulating film53 is formed thereon, and after a contact hole reaching the drain wiringline 52 is formed, a pixel electrode 54 made of a transparent conductivefilm is formed. The organic EL element portion is formed of the pixelelectrode 54, an organic EL layer 55 formed over the pixel electrode andthe third interlayer insulating film 53, a first electrode 56 formed onthe organic EL layer and made of MgAg compound, and a second electrode57 made of Al. Although not shown, if a color filter is provided, colordisplaying can also be made. In any event, if the method of fabricatingthe active matrix substrate shown in the examples 1 to 5 is applied, theactive matrix type EL display device can be easily fabricated. Such anactive matrix type EL display device can be fabricated by using anactive matrix substrate fabricated by freely combining the examples 1 to3.

Embodiment 6

An active matrix substrate, a liquid crystal display device, and an ELdisplay device fabricated by carrying out the present invention can beused for various electro-optical devices. Further, the present inventioncan be applied to all electronic instruments incorporating suchelectro-optical devices as display media. As the electronic instrument,a personal computer, a digital camera, a video camera, a portableinformation terminal (mobile computer, portable telephone, electronicbook, etc.), a navigation system, and the like can be enumerated.

FIG. 23A shows a portable information terminal which is constituted by amain body 2201, an image input portion 2202, an image receiving portion2203, an operation switch 2204, and a display device 2205. The presentinvention can be applied to the display device 2205 and other signalcontrol circuits.

Such a portable information terminal is often used outdoors as well asindoors. In order to enable a long time use, a reflection type liquidcrystal display device which does not use a backlight but uses outerlight is suitable as a low power consumption type. However, in the casewhere the environment is dark, a transmission type liquid crystaldisplay device provided with a backlight is suitable. From such abackground, a hybrid liquid crystal display device provided withcharacteristics of both the reflection type and the transmission typehas been developed. The present invention can also be applied to such ahybrid liquid crystal display device. The display device 2205 isconstituted by a touch panel 3002, a liquid crystal display device 3003,and an LED backlight 3004. The touch panel 3002 is provided tofacilitate the operation of the portable information terminal. In thestructure of the touch panel 3002, a light emitting element 3100 such asan LED is provided at one end, a light receiving element 3200 such as aphotodiode is provided at the other end, and an optical path is formedtherebetween. When this touch panel 3002 is pressed to block the lightpath, the output of the light receiving element 3200 is changed. Thus,when this principle is used and the light emitting element and the lightreceiving element are arranged in matrix form on the liquid crystaldisplay device, the panel can be made to function as an input medium.

FIG. 23B shows a structure of a pixel portion of a hybrid liquid crystaldisplay device. A drain wiring line 177 and a pixel electrode 178 areprovided on an interlayer insulating film on the pixel TFT 204 and theholding capacitance 205. Such a structure can be formed by applying theexample 4. The drain wiring line is made of a laminate structure of a Tifilm and an Al film, and is made a structure serving also as a pixelelectrode. The pixel electrode 178 is formed by using a transparentconductive film material explained in the example 4. When the liquidcrystal display device 3003 is fabricated from such an active matrixsubstrate, it can be preferably used for a portable informationterminal.

FIG. 24A shows a personal computer which is constituted by a main body2001 provided with a microprocessor, memory and the like, an image inputportion 2002, a display device 2003, and a keyboard 2004. The presentinvention can be applied to the display device 2003 and other signalprocessing circuits.

FIG. 24B shows a video camera which is constituted by a main body 2101,a display device 2102, an audio input portion 2103, an operation switch2104, a battery 2105, and an image receiving portion 2106. The presentinvention can be applied to the display device 2102 and other signalcontrol circuits.

FIG. 24C shows a goggle type display which is constituted by a main body2901, a display device 2902, and an arm portion 2903. The presentinvention can be applied to the display device 2902 and other not-shownsignal control circuits.

FIG. 24D shows an electronic play equipment, such as a TV game or videogame, which is constituted by an electronic circuit 2308 such as a CPU,a main body 2301 mounted with a recording medium 2304 or the like, acontroller 2305, a display device 2303, and a display device 2302incorporated in the main body 2301. The display device 2303 and thedisplay device 2302 incorporated in the main body 2301 may display thesame information. Alternatively, the former is made a main displaydevice, and the latter is made a sub display device which displaysinformation of the recording medium 2304, displays the operation stateof the equipment, or can also be made an operation plate by adding thefunction of a touch sensor. Besides, the main body 2301, the controller2305, and the display device 2303 may be connected with wiredcommunication to mutually transmit signals, or may be connected withwireless communication or optical communication by providing sensorportions 2306 and 2307. The present invention can be applied to thedisplay devices 2302 and 2303. A conventional CRT may be used for thedisplay device 2303.

FIG. 24E shows a player using a recording medium recording a program(hereinafter referred to as a “recording medium”), which is constitutedby a main body 2401, a display device 2402, a speaker portion 2403, arecording medium 2404, and an operation switch 2405. A DVD (DigitalVersatile Disc), compact-disk (CD), or the like is used as the recordingmedium, and reproduction of a music program, picture display,information display through a video game (or TV game) or the Internetcan be performed. The present invention can be preferably applied to thedisplay device 2402 and other signal control circuits.

FIG. 24F shows a digital camera which is constituted by a main body2501, a display device 2502, an eyepiece portion 2503, an operationswitch 2504, and an image receiving portion (not shown). The presentinvention can be applied to the display device 2502 and other signalcontrol circuits.

FIG. 25A shows a front type projector which is constituted by an opticalsource system and display device 2601 and a screen 2602. The presentinvention can be applied to the display device and other signal controlcircuits.

FIG. 25B shows a rear type projector which is constituted by a main body2701, an optical source system and display device 2702, a mirror 2703,and a screen 2704. The present invention can be applied to the displaydevice and other signal control circuits.

FIG. 25C is a view showing an example of the structures of the lightsource optical system and display devices 2601 and 2701 in FIG. 25A andFIG. 25B. Each of the light source optical system and display devices2601 and 2702 is constituted by a light source optical system 2801,mirrors 2802, and 2804 to 2806, a dichroic mirror 2803, a beam splitter2807, a liquid crystal display device 2808, a phase difference plate2809, and a projection optical system 2810. The projection opticalsystem 2810 is constituted by a plurality of optical lenses. AlthoughFIG. 25C shows an example of a three-plate system in which three liquidcrystal display devices 2808 are used, the invention is not limited tothis system, but a single plate optical system may be adopted. Besides,in light paths indicated by arrows in FIG. 25C, an optical lens, a filmhaving a polarizing function, a film for adjusting a phase, an IR filmor the like may be suitably provided.

FIG. 25D is a view showing an example of the structure of the lightsource optical system 2801 in FIG. 25C. In this example, the lightsource optical system 2801 is constituted by a reflector 2811, a lightsource 2812, lens arrays 2813 and 2814, a polarization conversionelement 2815, and a condensing lens 2816. Incidentally, the light sourceoptical system shown in FIG. 25D is merely an example, and the inventionis not limited to the structure shown in the drawing.

Besides, although not shown here, the present invention can also beapplied to a navigation system, a readout circuit of an image sensor,and so on. Like this, the scope of application of the present inventionis very wide, and the invention can be applied to electronic instrumentsof any fields. Besides, the electronic instruments of this example canbe realized by using the technique of the examples 1 to 5.

As described above, the crystalline semiconductor film of the presentinvention is obtained by the laser annealing method in which a pulsedoscillation type or continuous-wave excimer laser or argon laser is usedas a light source, and a linearly formed laser beam through an opticalsystem is irradiated to an island-like semiconductor layer from both thefront side and the reverse side of a substrate on which the island-likesemiconductor layer is formed. A laser apparatus used in such a laserannealing method does not require a complicated structure, but a mirrorhas only to be provided at the reverse side of the substrate. Thus, itcan easily meet the increase of the size of a processed substrate aswell.

Then, as described above, a crystalline semiconductor film in which theposition and size of a crystal grain is controlled can be obtained.Besides, by using such a crystalline semiconductor film for a channelformation region of a TFT, it is possible to realize the TFT capable ofoperating at high speed. Further, the invention can provide a techniqueby which such a TFT can be applied to various semiconductor devices suchas a transmission type liquid crystal display device or a display deviceusing an organic electroluminescence material.

What is claimed is:
 1. A method of fabricating a semiconductor device, comprising the steps of: providing a translucent substrate having first and second surfaces, the second surface opposed to the first surface; forming a base film of a first thickness over the first surface of the translucent substrate; forming a region of the first thickness and a region of a second thickness smaller than the first thickness by etching a part of the base film; forming an island-like semiconductor layer over the base film and over the region of the first thickness and the region of the second thickness; and crystallizing the island-like semiconductor layer by irradiating a laser beam to the island-like semiconductor layer from an upper portion of the island-like semiconductor layer over the first surface of the translucent substrate and from a side of the second surface of the substrate through the translucent substrate.
 2. A method of fabricating a semiconductor device according to claim 1, wherein a difference in film thickness between the region of the first thickness and the region of the second thickness is 30 to 100 nm.
 3. A method of fabricating a semiconductor device according to claim 1, wherein the laser beam irradiated to the island-like semiconductor layer from the other surface side is a laser beam having passed through the translucent substrate.
 4. A method of fabricating a semiconductor device in which a thin film transistor is provided over a translucent substrate having first and second surfaces, the second surface opposed to the first surface, the method comprising the steps of: forming a base film of a first thickness over the first surface of the translucent substrate; forming a region of the first thickness and a region of a second thickness smaller than the first thickness by etching a part of the base film; forming an island-like semiconductor layer over the base film and over the region of the first thickness and the region of the second thickness; crystallizing the island-like semiconductor layer by irradiating a laser beam to the island-like semiconductor layer from an upper portion of the island-like semiconductor layer over the first surface of the translucent substrate and from a side of the second surface of the substrate through the translucent substrate; and forming the thin film transistor so that at least a part of a gate electrode of the thin film transistor overlaps with the region of the first thickness.
 5. A method of fabricating a semiconductor device according to claim 4, wherein a difference in film thickness between the region of the first thickness and the region of the second thickness is 30 to 100 nm.
 6. A method of fabricating a semiconductor device according to claim 4, wherein the laser beam irradiated to the island-like semiconductor layer from the other surface side is a laser beam having passed through the translucent substrate.
 7. A method of fabricating a semiconductor device, comprising the steps of: providing a translucent substrate having first and second surfaces, the second surface opposed to the first surface; forming an island-like heat conduction layer over the first surface of the translucent substrate; forming a base film of a first thickness over the translucent substrate to cover the island-like heat conduction layer; forming an island-like semiconductor layer which is formed over the base film, which has an area larger than the island-like heat conduction layer, and at least a part of which overlaps with the island-like heat conduction layer; and crystallizing the island-like semiconductor layer by irradiating a laser beam to the island-like semiconductor layer from an upper portion of the island-like semiconductor layer over the first surface of the translucent substrate and from a side of the second surface of the substrate through the translucent substrate.
 8. A method of fabricating a semiconductor device according to claim 7, wherein the heat conduction layer is formed of at least one selected from the group consisting of aluminum oxide, aluminum nitride, and aluminum nitride oxide.
 9. A method of fabricating a semiconductor device according to claim 7, wherein the heat conduction layer is formed of a compound containing Si, N, O and M, M being at least one selected from the group consisting of Al and rare earth elements.
 10. A method of fabricating a semiconductor device in which a thin film transistor is provided over a translucent substrate having first and second surfaces, the second surface opposed to the first surface, the method comprising the steps of: forming an island-like heat conduction layer over the first surface of the translucent substrate; forming a base film of a first thickness over the translucent substrate to cover the island-like heat conduction layer; forming an island-like semiconductor layer which is formed over the base film, which has an area larger than the island-like heat conduction layer, and at least a part of which overlaps with the island-like heat conduction layer; crystallizing the island-like semiconductor layer by irradiating a laser beam to the island-like semiconductor layer from an upper portion of the island-like semiconductor layer over the first surface of the translucent substrate and from a side of the second surface of the substrate through the translucent substrate; and forming the thin film transistor so that at least a part of a gate electrode of the thin film transistor overlaps with the island-like heat conduction layer.
 11. A method of fabricating a semiconductor device according to claim 10, wherein the heat conduction layer is formed of at least one selected from the group consisting of aluminum oxide, aluminum nitride, and aluminum nitride oxide.
 12. A method of fabricating a semiconductor device according to claim 10, wherein the heat conduction layer is formed of a compound containing Si, N, O and M, M being at least one selected from the group consisting of Al and rare earth elements. 